Its in an account which isn’t owned by any person - the account of a “contract”, a computer program made to accept money and send it out based on a set of rules. Unfortunately for this person, the program’s rules don’t let it send back out the kind of currency he added. It’s like if you tried to drop off Euros at an American bank and instead of telling you they don’t accept Euros, they put it in the cash register and, when you asked for it back, told you they can’t give out Euros. The question is whether someone can figure out how to modify the program to accept more currencies (even though the program was designed to be immutable).
The smart contract is a small bit of code (and storage) that lives on the blockchain. It lives at an address, hence why tokens can be sent to it.
Unfortunately, the smart contract has no way of transferring the tokens out, and it can't be upgraded. Therefore the tokens are locked in there forever.
It's basically in limbo. The recipient contract isn't setup to receive funds from that chain, but it is a valid address. The recipient from a quick scan of the content appears to be immutable so it can never be updated to "receive" the funds.
It was never 'there' to begin with, so it's not like that, really.
It's like he said, "Hey, let's play a game. Right now, everyone agrees I should have 50k quatloos to do something with. (That's not the game; that's a fact and the premise going into the game.) The game is this: I'll write a note, and let's all agree that ultimately I will no longer have 50k quatloos after I publish the note, okay?" In fact, it's so much like that, that that's pretty much what it is.
(Getting it "back" is a matter of getting everyone to agree that "okay, now you have the 50k quatloos again".)